The present invention relates to clean up operations in subterranean well bores, and more particularly, to surfactant wash treatment fluids that may be used in clean-up operations to render the well bore water-wet and/or remove oil wet solids in well bores that may be present as a result of using oil-based drilling or drill-in fluids or other fluids including aqueous-based fluids that may comprise an oily component. The compositions and methods of the present invention are applicable to both hydrocarbon-producing wells as well as to injection wells.
Oil-based fluids, e.g., drilling or drill-in fluids are often used in subterranean applications. The choice of oil-based or water based types of fluids is often driven by performance characteristics of each fluid, the type of well to be drilled, cost considerations and the characteristics of the oil or gas field in which the well is to be drilled as well as environmental concerns.
The primary potential benefits of selecting an oil-based fluid are thought to include: superior hole stability, especially in shale formations; well bores with less wash out potential, providing wells that are more in gauge with the bit; formation of a thinner filter cake than a filter cake achieved with a water-based fluid; excellent lubrication of the drill string and down hole tools; penetration of salt beds without sloughing or enlargement of the hole; greater rate of bit penetration; better tolerance to fluid contamination as well as other benefits that should be known to those skilled in the art. Alternatively, a water based fluid may be used that contains a substantial amount of an oily component(s) in order to achieve some of these same performance characteristics as oil based fluids. The primary potential benefits of using such a fluid are thought to include: faster rate of penetration and superior hole stability relative to other water based fluids. Potential drawbacks may include possible increased rheology, fluid incompatibility, lower hole stability relative to oil based fluids, and potential environmental concerns. Perhaps one of the main advantages of adding oil to a well bore fluid is to reduce the density of the fluid, which may be desirable in certain circumstances.
Oil-based fluids usually contain some water, either formed in the formulation of the drilling fluid itself, or residual water in the hole, or intentionally added water to affect the properties of the drilling fluid or mud. Those that contain intentionally added water are often in an emulsion form, and are often referred to as invert emulsions. In such emulsions, an emulsifier is usually included to stabilize the emulsion. Typical examples of such emulsifiers include polyvalent metal soaps, fatty acids, fatty acid soaps, fatty amides, and other similar suitable compounds.
An especially beneficial property of oil-based fluids is their excellent lubrication qualities. These lubrication properties permit the drilling of wells having a significant vertical deviation, as is typical in off-shore or deep water drilling operations or when a horizontal well is desired. In such highly deviated holes, torque and drag on the drill string can be a significant problem because the drill pipe lies against the low side of the hole, and the risk of pipe sticking is high when water based fluids are used. In contrast, oil-based fluids provide a thin, slick filter cake which helps to prevent pipe sticking, and thus the use of the oil-based fluid may be beneficial.
Despite the many advantages of utilizing oil-based fluids, they have been associated with several disadvantages. Generally speaking, the use of oil-based drilling fluids and fluids can have high initial and operational costs. These costs can be significant depending on the depth of the hole to be drilled. However, the higher costs may be justified by the improved performance of the oil based drilling fluid, which in addition to the previously mention benefits also may prevent the caving in or hole enlargement that can greatly increase drilling time. Use of oil based fluids can often result in lower overall well construction costs because of simplified drilling and reduced subsequent completion costs.
One of the main drawbacks of using oil-based fluids in the hydrocarbon bearing portion of the well-bore is that the filter cake usually should be cleaned up for optimum production to occur. For proper clean up so as to afford the greatest hydrocarbon production, the filter cake preferably needs to be removed and the formation solids should be left water-wet. To clean up the well bore when an oil based fluid has been used, the oil-wet filter cake should be treated in some manner that results in converting the oil-wet solids contained in the filter cake to water-wet solids, which can then be treated and removed from the well bore. The resultant well bore should be rendered water-wet. Production can then occur. Production after a clean up is often thought to be superior to production without filter cake clean up.
One method that has been developed to clean up oil-wet solids that remain in the well bore as a result of using an oil-based fluid is a two-step process involving a mutual solvent treatment followed by an acid treatment. Using a mutual solvent to clean up oil-wet solids that result from using an oil based fluid is often thought to be necessary prior to acid treatment to avoid the formation of an undesirable residue that is extremely viscous and that can cause more damage from emulsion blockage than not doing the clean up in the first place. In such treatments, a solvent that is soluble in oil and water usually is used to wash away the oil from the solids so that the solids can then be dissolved or broken down by an acid treatment. Examples of suitable mutual solvents may include isopropanol or EGMBE. Washing the solids with the mutual solvent is thought to enable the acid to interact with the solids to degrade them.
There are several disadvantages associated with this process. First, it is a multi-step process, thus involving additional time and expense. Second, the mutual solvent may present environmental considerations and handling concerns for personnel. Third, the process can be time-consuming because the time needed for the solvent to change the wettability of the solids may take days of rig time. Additionally, when a strong acid is used after the mutual solvent, often the acid will not be evenly distributed in the well bore, and the result will be incomplete clean up because the acid will be spent in localized areas where it contacts the formation rather than uniformly attacking the now water-wet solids. Also, there are some concerns that this type of process may not be effective generally. For instance, the long-term effects on productivity are unknown.